Prior arts apparatus and methods have addressed the need for drying feedstock with emphasis on drying municipal solid waste for energy conversion. Economic utilization of the waste with ecologically balanced solution is yet to be found.
Mechanical and biological treatment (MBT) process is a well established process for municipal solid waste (MSW) treatment in Europe, but its environmental footprint and economical viability pose serious questions. This is so, due to the fact that the process feed is a mixed waste (bulk collected waste) with high loads of heavy metals and persistent organic pollutants. To strip the contaminants from the recyclables or the organic fraction, if not an impossible task, it is an extremely costly one. For these reasons as well as environmental guidelines, the end users are reluctant to use MBT products.
A by-product of waste water treatment is a solid waste referred to as sludge, which accounts for over 40% of waste water stream. The common practice for treatment of sludge is composting with maturing time of over 50 days, which is not a practical technology for large quantities of sludge. The caloric value of the compost is also very low due to complete mineralization of the waste. Instead, a low grade heat process along with technology such as exothermic heat generated by microorganisms are capable of removing water at the lowest possible residence time and minimal biodegradation, hence preserving maximum caloric value of the waste. Prior art devices and methods have prioritized drying of waste compared to composting. It has been suggested that composting for the most part has significant uncertainties, and it is a non-uniform process (U.S. Pat. Nos. 3,419,377; 4,956,002; and 5,688,686).
Drying waste with maximum efficiency and minimum environmental impact requires a heat source on the order of 100° C. or less. This energy could be harvested from the exhausts of turbines or from condensers on a steam topping cycle. To be able to harness heat from these waste sources, a dryer with a specific design is required. Dryers such as bed dryers or drum dryers require a temperature in the range of 200° C. to 350° C. Use of such levels of heat reduces caloric value of the waste, due to partial combustion of the feedstock. Adjusting the driers to work at a temperature less than 100° C. requires a drastic change in the design and structure of the prior arts. Having a dryer to utilize these heat sources reduces or eliminates the use of fossil fuels in the process, which ultimately reduces the drying cost and establishes better environmental records.
Up to now, the art focus has been to increase the efficiency of low-grade heat, so that a maximum amount of useful work can be achieved from vapor cycle engines (U.S. Pat. No. 3,950,949 A; U.S. Pat. No. 6,333,445 B1; U.S. Pat. No. 6,763,680 B2; U.S. Pat. No. 7,278,264 B2, etc.). These procedures describe a method which enhances the efficiency of the vapor cycle engines using low-grade heat sources. The efficiency of engines using low-grade heat is normally half that of engines run by primary fuel sources. Using heat exchangers may help to overcome this problem, but it adds to the energy and capital cost. So, although the efficiency of low-grade heat at or below 85° C. for the production of power is too low, it is suitable for the drying of feedstock specifically if it is used with a technology that guarantees the continuity of the heating source. Employing low-grade heat at less than 100° C. could preserve the caloric value of feed stock. U.S. Pat. No. 4,888,885, U.S. Pat. No. 6,163,981 and U.S. Pat. No. 6,742,284, B2 disclose technologies using low-grade heat sources for drying feedstock. These dryers are indeed modified versions of bed or rotary drum dryers which are using low grade heat. The major drawback of these dryers is their inadequate air distribution system, where moist air condenses on the feedstock as it finds its way out.
Biological action in feedstock, in a given condition, generates heat, which can be harvested similar to low-grade heat process. A number of attempts have been made to use this energy for the production of combustible solid fuel from organic waste (U.S. Pat. No. 8,124,401 B2, U.S. Pat. No. 7,960,165 B2, U.S. Pat. No. 7,662,205 B2, U.S. Pat. No. 7,744,671 B1, U.S. Pat. No. 4,837,153, EP 2000449 A1). The prior art discloses methods and techniques using exothermic heat for composting or drying of microorganism-rich feedstock from 65% moisture content to less than 30% in a period of 3-20 days. U.S. Pat. No. 8,124,401 B2 is capable of producing biofertilizer from sludge through harvesting the heat from the bioreactor to feed in a drying unit with assist of forced airflow system. Although the system adequately dries the sludge it exhibits serious disadvantageous. Drying uniformity has a direct influence on drying efficiency. The airflow system in the prior arts introduces air in only one direction. The airflow in the drying chamber in the process of passing through the sludge cools off, causing non-uniform drying of the sludge. Reversal of airflow direction results in the drying uniformity, as well as a reduction in drying time. U.S. Pat. No. 7,960,165 B2 is also suffering from the same drawbacks. Most importantly, the latter prior arts' process (U.S. Pat. No. 8,124,401 B2, U.S. Pat. No. 7,960,165 B2) require costly process equipment and an elevated operational cost. Although European patent (EP 2000449 A1) aerates waste alternatively from above and below the dryer cells which promotes drying uniformity, the drying process is yet inefficient due to the fact that the air condenses if the travelling distance of hot air in a moist environment exceeds 1.5 meters. This finding has been estimated through thermodynamic calculations and tested in the process of ongoing work. The airflow restriction as such translates itself into a shorter drying period of cells in a vertical direction, consequently larger space requirement in the transverse direction. The prior arts are also extremely inefficient due to the fact that the parameters to control the energy have not been well explored.
The forthcoming findings in the course of this invention well justify the preceding statement. The main controlling variable for the dryer is the outlet relative humidity. Controlling the drying process using the outlet relative humidity may be explained by the following example: Set the outlet relative humidity of each zone at a specific number, say at 95%. Once the outlet relative humidity exceeds this number in one or several zones, the automation system sends a message to the control room that the drying gas (air, CO2 or the like) flow rate should be increased, thus increasing the efficiency of the gas flow rate. This is the response of the dryer at the beginning of the drying procedure, where the feedstock is rich in free water. Towards the end of the drying process, the moisture source in the feedstock at the drying apparatus is the water generated through diffusion. The diffusion process is slow, so the outlet relative humidity will drop below the set point. To remove the moisture efficiently, the gas flow rate should be reduced. This also minimizes fine dried particles to be carried over by exhaust gases. These controls bring about substantial savings in energy.
Although the earlier arts in the field benefitted from using heat exchangers in the feedstock drying process (U.S. Pat. No. 8,124,401 B2, U.S. Pat. No. 7,960,165 B2) the proposed systems of using heat exchangers are relatively inefficient since the fact that the heat source to be recovered by heat exchangers is very low was left unnoticed. For instance, the heat generated by exothermic reactions is about 65° C. Recovering heat at such a range is very inefficient and demands an extensive surface area—recalling the fact that heat transfer to an object is a function of surface area. Therefore, heat loss or space requirement would be a critical issue in developing an energy-efficient drying technology.
The prior arts on drying are not configurable to different heat sources or different feedstocks. The importance of this invention also lies in its versatility concerning heat sources or feedstock diversity. The dryer works with all sorts of energy sources, such as low grade heat, solar heat, heat produced from exothermic reactions (self-heating) or heat generated from primary sources such as electricity or gas. Other examples of heat sources include but are not limited to unfocused solar energy, geothermal energy, ocean temperature gradients, process waste heat, exothermic biological heat, and heat harvested from incinerator or boiler exhaust.
A further addition to the prior arts is that the dryer in one embodiment may be used as a composter by varying the airflow rate to the zones, as well as recycling a portion of the exhaust gas in each zone to the inlet line of that zone which can be appreciated by one who is skilled in the art. This versatility of the present invention is valuable where prior arts were either in the form of a dryer or a composter per se. The composter—by benefiting from the features of the dryer that is automatically controlling and recycling airflow system, preserving moisture of the feedstock in different zones at optimum level and adjusting the heat of each zone through the heat exchangers system of the dryer—increases composting efficiency and also reduces residence time of the feedstock. These processes not only reduce energy costs, but also contribute in eliminating harmful gases. Uniform distribution of heat or airflow system prevents creation of compost pile hot spots or dead spots and thus guarantees that the end product of the process meets the required environmental standards.
Another benefit when compared with the prior arts is that this invention could also dry organic and inorganic components of the waste in a single run. In the absence of any source of low grade heat, the heat generated in the drying process of organic waste can be used also to dry the inorganic fraction of waste.
The invention proposed herewith is a vertical oriented dryer chamber perpendicular to its length with embodiments specific to feedstock as well as energy source. The feedstock continuously or in a connected series enters from the top zone and moves downwardly by gravity force, thus in general eliminating any need for an extra device for transporting the feedstock to the lower end. A custom-designed discharge system assists the uniform discharge of the dried feedstock from the opening at the lower end. This is another important addition to the prior art, where the discharge system contributes to the uniformity of drying through uniform discharging of the feedstock. The vertical oriented structure of the drying chamber along with its uniform forced air distribution—recycling efficiently recovered heat by the heat exchangers system embedded in the body of the drying chamber and uniform feedstock discharge—preclude the generation of hot spots and guarantee the feedstock's uniform drying. Recycling the recovered heat through the heat exchangers system reduces the residence time of a feedstock in the drying chamber and contributes to the reduction of harmful gases such as volatile organic compounds and the waste odor in the procedure. In addition, the dryer leaves a very small environmental footprint, since the filtration system treats discharged gases for hazardous emissions before sending it back to the dryer—which is equipped with a biofilter capable of trapping and neutralizing particulates and hazardous gases.